Dielectric lenses (which can be made from materials having refractive indices (n) varying from 2 to 9 or more) can be used to focus millimetre wave radiation onto antennae (dipoles or quadrupoles) which are fabricated on planar substrates. These antennae collect the radiation, which is detected and processed by active components fabricated as an integrated circuit on a slice of a semiconductor material, usually silicon but also possibly gallium arsenide. According to the theory taught by Brewitt-Taylor et al described in Electronics Letters (1981) 17 729-31, planar antennae fabricated upon a substrate of high refractive index (such as silicon) in contact with a medium of low refractive index (such as air) will exhibit their maximum sensitivity in a direction which is perpendicular to the plane of the substrate, and lies in the high refractive index medium, as is illustrated in FIG. 1 of the accompanying drawings. Hence, a possible structure which may be used with dielectric lenses of materials such as alumina (refractive index 3.1) is to fabricate the planar antennae as a photodefined array of evaporated metal bars on a slice of silicon, connected to a sensing and signal processing circuit manufactured in the silicon as illustrated in FIG. 2 of the accompanying drawings. The back of this circuit :s pressed into close contact with the alumina lens as shown in FIG. 3 of the accompanying drawings. The silicon and alumina must be :n very close contact, with a maximum permissible air gap of about 5 .mu.m.
We have found that the structure illustrated in FIG. 3 is not always feasible if the lens material possesses a considerably higher refractive index (n equals approximately 6) than the silico (silicon n approximately equal to 3.4) If this is the case, it is necessary for the high refractive index material of the lens to be directly in contact with the metallisation of the antenna structure. Only very small air-gaps are permissible if the device structure is to be viable (of the order of 1 micron or less) and a pressed-contact structure of the type shown in FIG. 3 presents considerable difficulties in fabrication. Nevertheless, higher refractive index lenses present a number of advantages in that the wavelength of the radiation (.lambda..sub.n) within the high refractive index is correspondingly shorter, as it is given by: EQU .lambda..sub.n =.lambda..sub.f /n
where .lambda..sub.f is the free space wave length, and n is the refractive index of the high refractive index material.
This means that, for a given frequency of operation, the antenna can be proportionately smaller, which in turn implies that a larger number of sensitive points can be fabricated per unit area of the array. Alternatively, the dielectric lens dimensions can also be reduced if a higher index material is employed thus resulting in a more compact system for a given array size.